The hypothalamic control of reproductive function is expressed through the receptor-mediated actions of GnRH on the pituitary gonadotroph. GnRH receptors in the pituitary gland exhibit prominent variations in number during the ovarian cycle and after changes in steroid feedback, and are modulated by the rate of GnRH secretion from the hypothalamus. In cultured pituitary cells, GnRH receptors undergo down-regulation during exposure to GnRH agonists, followed by a subsequent elevation of sites that is dependent on protein synthesis. GnRH antagonists do not cause receptor down-regulation, but high affinity antagonist analogs bind for extended periods to cause receptor occlusion and prolonged inhibition of GnRH action. Analysis of the rat pituitary GnRH receptor by photoaffinity labeling reveals two binding subunits fo Mr 53,000 and 42,000. The receptor-activated processes leading to gonadotropin secretion are highly calcium dependent, and are initiated by rapid phospholipid hydrolysis with production of arachidonic acid metabolites, diacylglycerol, and inositol phosphates. The role of protein kinase C in gonadotrophin secretion is indicated by the ability of phorbol esters and synthetic diacylglycerols to stimulate LH release, and by the redistribution of protein kinase C between cytosol and membrane fractions in GnRH-stimulated gonadotrophs. It is likely that the effects of arachidonate metabolites are integrated with those of calcium-calmodulin and calcium, phospholipid-dependent protein kinases during the immediate and sustained phases of GnRH-induced gonadotrophin secretion. Studies with dihydropyridine calcium channel agonist and antagonist derivatives, and measurements of cytoplasmic calcium in Quin-2-loaded cells, revealed that increases in intracellular calcium via voltage-sensitive calcium channels partially reproduce GnRH action, and suggest that GnRH causes activation of such channels in the gonadotroph. The increase in cytoplasmic calcium during GnRH action also originages in part from mobilization of internal calcium stores, and its relatively small magnitude is consistent with concomitant activation of protein kinase C as an intermediate step in GnRH action.